1. Field of the Invention
This invention relates to cordless telephones and more particularly to channel selection in cordless telephone systems.
2. Description of the Related Art
Cordless telephones include a wall powered base station and a cordless battery powered handset which communicate with each other over a predetermined frequency band through the use of radio transmissions. The frequency band is broken up into a number of channels that may be used by the handset and base station. The handset and base station may both be sharing the same channel (time division duplex) or transmitting on one channel and receiving on another (frequency division duplex). There may be, e.g., forty channels over which communication takes place between the base station and the handset. Having more than one channel increases the likelihood of finding a clear channel on which to communicate.
There can be multiple handsets and base station pairs in relatively close proximity to each other. It is necessary to prevent the multiple handsets and base stations from erroneously or illegally communicating with one another. Therefore, the handset and base station exchange unique security IDs so that they communicate only with the correct partner. The higher the number of bits in the security ID, the less likely the chances of a handset or base station being able to communicate with the incorrect unit. The unique pair of security IDs is created using a random number generator, which is typically implemented in software in both the handset and the base station. When the handset and the base station register with one another, e.g., when the handset is mated in the cradle of base station 101, they exchange security IDs and henceforth use these IDs to establish communications with each other. The random security IDs are stored in nonvolatile memory, so neither a power outage nor an electrostatic discharge event destroys them.
In a standby mode of operation in which most of the power of the handset is off, the handset periodically wakes-up to scan all the channels in the appropriate frequency band to see if there is an incoming call. In the U.S., telephones ring in a pattern of two seconds on and four seconds off. That means that the handset should scan all forty channels at least every two seconds to ensure that the handset does not miss the ring indication being transmitted from the base station over one of the channels. Scanning for incoming calls requires the handset to be actively powered on.
Thus, scanning all forty channels at least every two seconds costs power, which reduces the battery life of the handset, an obviously undesirable result.
In a time division duplex system, where the handset and base station transmit and receive on the same channel, the handset and base station may not always agree on which channels are free and which are not acceptable for communication because of interference. That is especially true if they are located a significant distance from one another, as interference levels may vary in different locations. In a frequency division duplex system, the handset and base station are not capable of measuring whether their counterpart's receive channel has interference or not. To speed initial linkup time between the handset and base station, it would be desirable for both the handset and the base station to know which channels are clear in both the transmit and receive direction. It would also be desirable to achieve longer battery life while still having the advantage of multiple channels.